Carbon fiber-reinforced composites are useful engineering materials having applications where there is a need for a combination of high strength and low weight. They comprise a matrix material reinforced with carbon fibers, and where the matrix material is itself carbon, the composite is generally referred to as a carbon-carbon composite, or simply "carbon-carbon." The carbon-carbon composites, when suitably coated or otherwise treated to protect them from oxidation, also exhibit excellent strength at high temperatures and consequently find uses in such applications as turbine engine components, structural airframe components, nose cones, exhaust nozzles, bearings, brake and clutch discs, pistons, furnace components, support structures, rotary pump vanes, and thermal and chemical resistant tubing, among others.
Carbon-carbon composites are made from carbon or graphite fibers in 1-, 2-, 3-, and n-dimensional (random fiber orientation) forms using tows, yarn, tape, or cloth which are impregnated with a resin. After forming and curing to the desired shape under heat and pressure, the shaped part is then pyrolyzed to decompose the resin matrix into carbon. Since some porosity usually remains, the part is typically reimpregnated and re-pyrolyzed until full density (about 1.6) is obtained. Finally, in applications where high strength at high temperatures is called for, oxidation resistance up to about 1400.degree. C. can be imparted by further treating with coatings and sealants such as silicon carbide, silica, and boron compounds. Joining such parts to other members typically involves carbon and graphite brazing methods which use reactive materials containing carbide-forming elements.
Useful as such materials may be, further improvement in the areas of high-temperature performance and their ability to be securely joined to themselves and to metals would be desirable in order to further expand their applications. For example, hypersonic vehicle and aerostructural applications will require materials capable of withstanding very high temperatures, with a suggested goal of 1600.degree.-2000.degree. C. in oxidizing atmospheres. Furthermore, since the above-mentioned brazing methods tend to produce weak and braze-starved joints and require special braze formulations which are not commercially available, improvements in joining carbon-carbon composites, such as called for in fusion reactor design among other applications, are greatly to be desired.